In wireless communication systems, mobility between different geographical areas is provided by allowing mobile stations to handoff their communications between base stations. The base station from which a mobile station is handing off from is typically referred to as a serving base station, and the base station to which the mobile station is to hand off to is typically referred to as a target base station. Mobile stations can include cellular telephones, personal digital assistants (PDAs), computers equipped with wireless transceivers (such as integrated transceivers or transceivers on PCMCIA cards), pagers, and the like.
There are two general types of handoff, hard handoff and soft handoff. Hard handoff is performed in a break-before-make manner, in which a mobile station ceases all traffic and control channel communications with the serving base station and then attempts to establish traffic and control channel communications with the target base station. This approach is used mostly to preserve air-link and network resources utilized by mobile stations in the handoff areas, and might be the preferred mode for high-speed packet data channels. It also simplifies the handling of data packets at the network infrastructure. However, hard handoff does not provide full diversity gain associated with soft handoff, thereby increasing the possibility of temporary session interruption. Such a temporary interruption would impact Quality of Service (QoS) for real-time applications, such as voice.
Using a soft handoff technique, a mobile station will simultaneously communicate with the serving and target base stations over both base stations traffic and control channels. This type of handoff provides diversity, as the mobile station can receive the same information from both base stations. Due to low chance of session interruption, soft handoff is typically preferred where real-time applications such as voice are involved. However, soft handoff requires synchronous resource allocation and scheduling, and coordinated packet handling on the network side. This has a direct impact on throughput efficiency and network complexity. Soft handoff also increases the required backhaul capacity, especially for high rate traffic channels. Additionally, it is only feasible when frequency reuse is 1:1 and strict traffic synchronization is required among all bases stations involved in the soft handoff. Moreover, soft handoff complicates packet scheduling and automatic retransmission request (ARQ) messages.
While many wireless communication technologies provide either soft or hard handoff, broadband communication technologies including cdma2000, 1xEV-DO, WCDMA, Flarion's Flash OFDM® and IEEE80216e provide both hard and soft handoff. Specifically, soft handoff is typically employed for communications that are sensitive to interruptions or errors, while hard handoff is employed for all other communications.